The difference between the two is the protein content-- all purpose has more and cake has less. The affects gluten development , which is why cakes and pastries make with cake flour have a softer texture- are easily bitten and cut, but messier! They are kind of interchangeable; for cakes it will be fine, but I wouldn't sub cake for AP if you were making something that needs a lot of structure (bread or pizza crust, for example).

Bread flour has less starch and more protein than cake flour. Eight parts (such as ounces or grams) of cake flour has the same thickening power as 10 parts of bread flour.
Bread flour frequently is used for general cooking purposes in commercial kitchens even though it has less thickening power than cake flour or pastry flour.

1. Bread flour is a strong flour used for making breads, hard rolls, and any product that requires high gluten.The best bread flours are called patents. Straight flours are also strong flours.
2. Cake flour is a weak or low-gluten flour made from soft wheat. It has a soft, smooth texture and a pure white color. Cake flour is used for cakes and other delicate baked goods that require low gluten content.
3. Pastry flour is lower in gluten than bread flour but higher than cake flour. It has the same creamy white color as bread flour,not the pure white of cake flour.Pastry flour is used for cookies,pie pastry,some sweet yeast doughs,biscuits,and muffins.

All-purpose flour,seen in retail markets,is not often found in bakeshops.This flour is formulated to be slightly weaker than bread flour so it can be used for pastries as well.

Bread flour has a protein content of 12-12.5%. That of all-purpose flour is 10-10.5%. Pastry flour's protein content is around 9%, and that of cake flour is the lowest at 7%. Yes, you can certainly substitute cake flour for AP flour in many baking applications - muffins, quick breads, and obviously cakes. Only in the case of the latter, cakes, would I suggest using entirely cake flour; for muffins, quick breads, and even some cookies, I typically use a ratio of 75% AP flour and 25% cake flour. While it is possible, with some true diligence, to find unbleached cake flour, most of it is bleached, resulting in the white color that has become the acceptable standard for cake flour. As has been suggested, it is important to sift the flour(s) of anything containing any amount of cake flour; it is so finely milled that it is notoriously clumpy. Swishing a whisk through it isn't going to do you any good. If you don't sift, you'll be tempted to overbeat your mixture to rid it of lumps, which will result in toughening it. Use any sort of sieve, and simply push through any remaining lumps with your fingers.